1. Field of the Invention
The present invention is related to planar broadband antennas and, more particularly, to an antenna for transmitting or receiving circularly-polarized signals.
2. Description of the Prior Art
Antennas producing circularly-polarized signals are known in the art. U.S. Pat. No. 5,861,848, issued to Iwasaki, for example, discloses a circularly polarized wave patch antenna with short circuit portion. The directivity of a patch antenna can be increased by incorporation of a choke ring ground plane, but this increases the weight of the antenna.
U.S. Pat. No. 5,815,122, issued to Nurnberger et al., for example, discloses a slot spiral antenna with a single spiral slot on one side of the antenna, and a spiral microstrip feed line on the reverse side. The reference teaches primarily a single-slot configuration which results in an antenna having a low directivity. Moreover, the placement of an additional component, such as a low-noise amplifier, on the antenna itself is impractical.
While the art describes planar antennas producing circularly-polarized radiation, there remains a need for improvements that offer advantages and capabilities not found in presently available devices, and it is a primary object of this invention to provide such improvements. It is another object of the invention to provide such a planar antenna with an improved directivity.
It is yet another object of the present invention to provide a slot array antenna having a distribution feed line which matches the input/output signals with the spatial angular configuration of the antenna slots.
It is further another object of the present invention to provide such a planar antenna which allows for the mounting of active circuitry on the antenna substrate.
Other objects of the invention will be obvious, in part, and, in part, will become apparent when reading the detailed description to follow.
A planar antenna includes a nonconductive substantially planar substrate and a transmission line disposed on one surface, a segment of the transmission line forming an arc of radius R centered on the antenna axis. A conductive layer on the other antenna surface includes two or more slotted openings, each slotted opening having one end located within a distance R of the antenna axis, such that, when an electromagnetic signal is fed into one end of the transmission line, electromagnetic energy is sequentially coupled into the slotted openings, and a circularly-polarized signal is radiated from the antenna substantially in the direction of the antenna axis. The electrical phase length of the transmission line is matched to the spatial angular difference between two consecutive slotted openings, so as to provide for a phased-array operation.
An amplifier or a connector may be electrically connected to one or both ends of the transmission line, or one end of the transmission line may be terminated in an impedance load to form a leaky-wave antenna. The slotted openings may comprise either or both straight and curved segments, and may be of the same or unequal lengths. Curved slotted openings may be oriented clockwise or counter-clockwise to transmit or receive either left-handed or right-handed polarized signals.